WO2008017930A1

WO2008017930A1 - Exhaust emission control system of internal combustion engine

Info

Publication number: WO2008017930A1
Application number: PCT/IB2007/002274
Authority: WO
Inventors: Masaaki Sato
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-08-08
Filing date: 2007-08-07
Publication date: 2008-02-14
Also published as: JP2008038806A

Abstract

An exhaust emission control system of an internal combustion engine having a plurality of cylinders and capable of operating in a reduced-cylinder operating mode in which fuel is burned in a reduced number of cylinders includes a storage-reduction type NOx catalyst operable to adsorb NOx and have the stored NOx reduced in the presence of a reducing agent, and a desulfurizing device that operates the engine in the reduced-cylinder operating mode (S103) when a desulfurization process is performed on the NOx catalyst. Owing to the reduce -cylinder engine operation, the amount of fuel supplied to each cylinder in which the fuel is burned is increased, and a combustion gas having a high temperature and a low air/fuel ratio is discharged, whereby the temperature of the NOx catalyst is prevented from being reduced. Accordingly, the NOx catalyst can be regenerated from sulfur poisoning even when the load of the engine is low.

Description

EXHAUST EMISSION CONTROL SYSTEM OF INTERNAL COMBUSTION

ENGINE

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to an exhaust emission control system of an internal combustion engine.

2. Description of the Related Art [0002] It is known to place a storage-reduction type NOx catalyst

(hereinafter simply referred to as "NOx catalyst") in an exhaust passage of an internal combustion engine. The NOx catalyst adsorbs and stores NOx contained in exhaust gas when the exhaust gas flowing into the catalyst contains a high concentration of oxygen, and releases the stored NOx while reducing the NOx in the presence of a reducing agent when the oxygen concentration of the exhaust gas flowing into the catalyst is reduced.

[0003] In the meantime, the NOx catalyst adsorbs and stores a sulfur component contained in the fuel, as well as the NOx. The sulfur component thus stored is less likely to be released from the NOx catalyst, as compared with the NOx, and tends to be accumulated within the NOx catalyst. This condition is called "sulfur poisoning". Since the sulfur poisoning causes a reduction in the NOx conversion efficiency of the NOx catalyst, there is a need to perform a desulfurization process for regenerating the NOx catalyst from sulfur poisoning (i.e., releasing the sulfur component from the NOx catalyst) at appropriate times. The desulfurization process is performed under certain conditions including, for example, that the NOx catalyst has a high temperature and that exhaust gas having a stoichiometric ratio or a rich air/fuel ratio is caused to flow through the NOx catalyst.

[0004] Here, a control technology for VHype engines as disclosed in, for example, Japanese Patent Application Publication No. 9-291814 (JP-A'9-291814) is known with which, during a desulfurization process, the air/fuel ratio of air-fuel mixtures fed into cylinders of one of the two banks is controlled to be rich or equal to the stoichiometric ratio, while the air/fuel ratio of air-fuel mixtures fed into cylinders of the other bank is controlled to be lean. References are also to be made to Japanese Patent Application Publication No. 8-61052 (JP-A-8-61052) and Japanese Patent Application Publication No. 8-189388 (JP-A-8- 189388).

[0005] According to the above technology, unburned substances are contained in the exhaust gas discharged from the cylinders of the above-indicated one bank, and the sulfur component released from the storage-reduction type NOx catalyst is reduced by the unburned substances. At the same time, the other bank is operated in a lean condition, which leads to a reduction in the fuel consumption of the engine as a whole. It is thus possible to perform the desulfurization process while assuring a reduced fuel consumption of the engine as a whole. [0006] It is, however, to be noted that a high temperature and a low air/fuel ratio are required for accomplishing the desulfurization, and the NOx catalyst needs to be kept exposed to a high-temperature atmosphere having a low air/fuel ratio for several minutes. If the engine runs at a low load, the exhaust gas discharged from the engine has a low temperature and a lean air/fuel ratio. In these operating conditions, the desulfurization of the NOx catalyst has to be discontinued.

[0007] Even if the desulfurization process is started while the load of the engine is relatively high, it becomes difficult to continue the desulfurization if the load is reduced during operation. In this case, if the desulfurization process is interrupted or stopped, there arises a need to perform the desulfurization process again, resulting in an increased amount of fuel consumption and deterioration of the fuel economy. If, on the other hand, the desulfurization process cannot be carried out for a long period of time, the NOx conversion efficiency is reduced. SUMMARY OF THE INVENTION

[0008] The invention has been developed in view of the above -de scribed situations. Thus, the invention provides an exhaust emission control system of an internal combustion engine in which a storage-reduction type NOx catalyst can be regenerated from sulfur poisoning even when the load of the engine is low.

[0009] According to one aspect of the invention, an exhaust emission control system of an internal combustion engine having a plurality of cylinders and capable of operating in a reduced-cylinder operating mode in which a fuel is burned in a reduced number of the cylinders is provided which includes a storage-reduction type NOx catalyst operable to adsorb and store NOx and reduces the stored NOx in the presence of a reducing agent, and a desulfuiϊzing device that operates the engine in the reduced-cylinder operating mode when a desulfurization process for regenerating the storage-reduction type NOx catalyst from sulfur poisoning is performed. [OOIO] Here, the internal combustion engine to which the invention is applicable is of a type capable of operating in an all-cylinder operating mode in which the fuel is burned in all of the cylinders, and also in a reduced-cylinder operating mode in which burning of the fuel is stopped in selected ones of the cylinders while the fuel is burned in the rest of the cylinders. [OOll] If the engine is operated in the reduced-cylinder operating mode, torque is not produced from the cylinders (hereinafter referred to as "deactivated cylinders") in which burning of the fuel is stopped, and therefore, the torque produced by the engine as a whole is reduced. If, however, the torque produced from the cylinders (hereinafter referred to as "activated cylinders") in which the fuel is burned is increased, the reduction of the torque produced by the engine as a whole may be eliminated. By supplying an increased amount of fuel to the activated cylinders, the torque produced in the activated cylinders can be increased. Namely, the increase in the torque produced in the activated cylinders can make up for the reduction in the torque produced in the deactivated cylinders. For example, the driver depresses the accelerator pedal by a larger degree in response to the reduction in the torque produced by the engine as a whole due to the reduced-cylinder engine operation, so that the amount of fuel supplied to the activated cylinders is increased. [0012] By supplying the increased amount of fuel to the activated cylinders in the above manner, the air/fuel ratio in the activated cylinders is reduced, and the combustion temperature is increased. Namely, the air/fuel ratio and temperature of the exhaust gas can be made close to those suitable for desulfurization of the NOx catalyst. Thus, the de sulfur ization process can be implemented in an expanded operating region of the engine, and the fuel efficiency or fuel economy is less likely to deteriorate or prevented from deteriorating.

[0013] The above -indicated desulfurizing device may determine the number of the cylinders to be deactivated in the reduced-cylinder operating mode so that the temperature and air/fuel ratio of the exhaust gas discharged from the engine are controlled to those required for releasing the sulfur component from the storage-reduction type NOx catalyst.

[0014] In the reduce-cylinder operating mode, the number of the cylinders to be deactivated may be equal to one half of all the cylinders, and burning of the fuel may be stopped in these cylinders (i.e., half of all the cylinders). [0015] In one embodiment of the invention, the internal combustion engine is a V-type eight-cylinder engine in which the fuel is burned at least once at intervals of 90° crank angle, and the desulfurizing device stops burning of the fuel in one half of the cylinders while permitting the burning in the other half of the cylinders at intervals of 180° crank angle when performing the desulfurization process.

[0016] In a large-displacement engine, such as a V-type eight-cylinder engine, the maximum torque that can be produced by the engine is larger than that of a small-displacement engine, and therefore, the large-displacement engine is more likely to be used under low-load conditions. It follows that the engine is likely to be operated at a lean air/fuel ratio. Accordingly, the large -displacement engine is more likely to encounter situations where it is difficult to perform the desulfurization process, as compared with the small-displacement engine.

[0017] The V-type eight-cylinder engine in which the fuel is burned at least once at intervals of 90° crank angle is able to operate with stability even if burning of the fuel is stopped in every other cylinder. Namely, even if the fuel is burned in every other cylinder, combustion takes place at equal intervals, thus assuring stable operating conditions of the engine. By burning the fuel at intervals of 180° crank angle, the engine can easily provide exhaust gas having a high temperature and a low air/fuel ratio during the desulfurization process. Thus, the desulfurization process can be implemented even in a large -displacement engine during low-load running. It is to be understood that other types of internal combustion engines than the V-type eight-cylinder engine may be operated with stability, in a manner similar to the V-type eight-cylinder engine, provided that the fuel can be burned at equal intervals.

[0018] In another embodiment of the invention, the exhaust emission control system further includes a variable valve actuating mechanism capable of keeping exhaust valves in closed positions, and the variable valve actuating mechanism keeps exhaust valves of the deactivated cylinders (in which burning of the fuel is stopped) in the closed positions when the desulfurizing device performs the desulfurization process while operating the engine in the reduced-cylinder operating mode.

[0019] If the exhaust valves of the deactivated cylinders are opened and closed in the same manner as in normal operation, air that was not used for burning the fuel is discharged into the exhaust passage. If the air is mixed with exhaust gas emitted from the activated cylinders, the air/fuel ratio of the exhaust gas from the activated cylinders is increased. Namely, even though the activated cylinders discharge exhaust gas having a reduced air/fuel ratio, the air/fuel ratio is increased due to the exhaust gas from the deactivated cylinders. As a result, the exhaust gas, when reaching the storage-reduction type NOx catalyst, has an undesirably high air/fuel ratio. It is also to be noted that the exhaust gas from the deactivated cylinders has a low temperature since no combustion takes place in these cylinders. Accordingly, the exhaust gas from the deactivated cylinders causes a reduction in the temperature of the exhaust gas when reaching the storage-reduction type NOx catalyst.

[0020] If the exhaust valves of the deactivated cylinders are kept closed, on the other hand, air is inhibited from being discharged from the deactivated cylinders into the exhaust passage, which makes it possible to feed exhaust gas having a higher temperature and a lower air/fuel ratio into the storage -reduction type NOx catalyst. Namely, the NOx catalyst can be regenerated from sulfur poisoning when the engine runs at a low load.

[0021] In a further embodiment of the invention, the desulfurizing device operates the engine in the reduced-cylinder operating mode when the load of the engine becomes equal to or lower than a first threshold value.

[0022] At the load equal to or lower than the first threshold value, the engine is in operating conditions in which the air/fuel ratio of the exhaust gas is increased to such a large value or the temperature of the exhaust gas is reduced to such a low level that it becomes difficult to accomplish the desulfurization process without operating the engine in the reduced-cylinder operating mode. Namely, the engine is operated in the reduced-cylinder operating mode when it becomes difficult to perform the desulfurization process. With this arrangement, the engine is not operated in the reduced-cylinder operating mode when the engine runs at a high load, and therefore, the exhaust air/fuel ratio is prevented from being excessively reduced while the exhaust temperature is prevented from being excessively increased.

[0023] In a still another embodiment of the invention, the desulfurizing device stops operating the engine in the reduced-cylinder operating mode when the load of the engine becomes equal to or higher than a second threshold value that is higher than the first threshold value during operation of the engine in the reduced-cylinder operating mode.

[0024] The second threshold value is set to be higher than the first threshold value because the exhaust temperature is reduced when the engine switches from the reduced-cylinder operating mode to the all-cylinder operating mode, and the exhaust temperature at the time of switching needs to be kept from being reduced to be lower than a temperature level required for the desulfurization process. If the engine is switched to the all-cylinder operating mode when the load becomes sufficiently high, the exhaust air/fuel ratio is prevented from being excessively reduced and the exhaust temperature is prevented from being excessively increased when the engine runs at a high load.

[0025] The exhaust emission control system of the internal combustion engine according to the invention is able to regenerate the storage -reduction type NOx catalyst from sulfur poisoning (namely, release the sulfur component from the NOx catalyst) even when the engine runs at a low load.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The features, advantages, and technical and industrial significance of this invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings in which^:

FIG. 1 is a view schematically showing the construction of an internal combustion engine and its exhaust system according to one embodiment of the invention! FIG. 2 is a view indicating the order of combustion in the cylinders of the engine according to the embodiment of FIG. Ii and

FIG. 3 is a flowchart illustrating a control routine of a desulfurization process performed in the embodiment of FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] In the following description and the accompanying drawings, the present invention will be described in more detail with reference to exemplary embodiments. [0028] Initially, one embodiment of the invention will be described in detail.

FIG. 1 schematically shows the construction of an internal combustion engine 1 and its exhaust system according to the present embodiment. The internal combustion engine 1 as shown in FIG. 1 is a V-type, eight-cylinder, fourcycle diesel engine. In the engine 1, an air-fuel mixture is burned in any of the cylinders at regular intervals of 90° crank angle, namely, each time the crankshaft rotates by 90 degrees.

[0029] The engine 1 includes a right bank 2 and a left bank 3. Each of the right bank 2 and the left bank 3 has four cylinders 4. Namely, the right bank 2 has No. 1, No. 3, No. 5 and No. 7 cylinders (denoted as #1, #3, #5 and #7, respectively, in FIG. l), and the left bank 3 has No. 2, No. 4, No. 6 and No. 8 cylinders (denoted as #2, #4, #6 and #8, respectively, in FIG. 1). A right-hand exhaust manifold 5 is connected to the right bank 2 such that the respective cylinders 4 of the right bank 2 are connected to the right-hand exhaust manifold 5. A left-hand exhaust manifold 6 is connected to the left bank 3 such that the respective cylinders 4 of the left bank 3 are connected to the left-hand exhaust manifold 6. The right-hand exhaust manifold 5 is connected at the other end to a right-hand exhaust pipe 7, and the left-hand exhaust manifold 6 is connected at the other end to a left-hand exhaust pipe 8.

[0030] A right-hand storage-reduction type NOx catalyst 9 (hereinafter referred to as "right-hand NOx catalyst 9") is provided at some midpoint in the right-hand exhaust pipe 7, and a left-hand storage-reduction type NOx catalyst 10 (hereinafter referred to as "left-hand NOx catalyst 10") is provided at some midpoint in the left-hand exhaust pipe 8. Each of the right-hand NOx catalyst 9 and left-hand NOx catalyst 10 has the functions of adsorbing and storing NOx contained in exhaust gas when the exhaust gas flowing into the catalyst contains a high concentration of oxygen, and reducing the stored NOx in the presence of a reducing agent when the exhaust gas flowing into the catalyst contains a low concentration of oxygen. [0031] The engine 1 is also provided with a variable valve actuating mechanism 11 capable of closing exhaust valves 15 provided in the respective cylinders 4, and keeping the exhaust valves 15 in the closed positions.

[0032] The engine 1 constructed as described above is equipped with an

ECU 13 as an electronic control unit for controlling the engine 1. The ECU 13 is adapted to control the operating state of the engine 1 in accordance with operating conditions of the engine 1 and driver's demands.

[0033] Various sensors, including a crankshaft position sensor 14 and an acceleration stroke sensor 17, are connected via electrical wiring to the ECU 13.

The crankshaft position sensor 14 produces a signal responsive to the crank angle of the engine 1, and the acceleration stroke sensor 17 produces an electric signal responsive to the amount of depression of an accelerator pedal 16 by the driver, and is thus capable of detecting an engine load. The output signals of these sensors are transmitted to the ECU 13. The load of the engine 1 is detected by the acceleration stroke sensor 17. [0034] On the other hand, the ECU 13 is connected via electrical wiring to the variable valve actuating mechanism 11 so that the variable valve actuating mechanism 11 is controlled by the ECU 13.

[0035] FIG. 2 shows the order of combustion in the cylinders 4 of the engine 1. In the right bank 2, combustion is caused to take place in the order of No. 1 (#1) - No. 7 (#7) - No. 3 (#3) - No. 5 (#5). In the left bank 3, combustion is caused to take place in the order of No. 2 (#2) - No. 4 (#4) - No. 6 (#6) - No. 8 (#8).

In the engine 1 as a whole, combustion is caused to take place in the order of No. 1

(#1) - No. 2 (#2) - No. 7 (#7) - No. 3 (#3) - No. 4 (#4) - No. 5 (#5) - No. 6 (#6) - No. 8

(#8). [0036] In the present embodiment, when a desulfurization process for regenerating the NOx catalyst 9, 10 from sulfur poisoning is performed while the engine 1 is operating at a load lower than a predetermined load, the engine 1 is operated in a reduced-cylinder operating mode in which the fuel is burned in a reduced number of cylinders 4 (i.e., less than all the cylinders 4). Namely, combustion is caused to take place at intervals of 180° crank angle in the reduced-cylinder operating mode, while combustion takes place at intervals of 90° crank angle in normal operation of the engine. More specifically, the fuel is burned in the No. 1, No. 7, No. 4 and No. 6 cylinders, whereas supply of the fuel to the No. 2, No. 3, No. 5 and No. 8 cylinders is stopped so that combustion is stopped in these cylinders. Alternatively, in contrast to the above, the fuel is burned in the No. 2, No. 3, No. 5 and No. 8 cylinders, whereas supply of fuel to the No. 1, No. 7, No. 4 and No. 6 cylinders is stopped so that combustion is stopped in these cylinders. [0037] In the present embodiment, control operations as described below in paragraphs (l) through (4) are performed.

[0038] (l) The engine 1 is operated in the reduced-cylinder operating mode when the load of the engine 1 is reduced to a threshold value (which will be called "first threshold value") or lower during the desulfurization process. At the load equal to or lower than the first threshold value, the engine 1 is in operating conditions in which the air/fuel ratio of the exhaust gas is increased to such a large value or the temperature of the exhaust gas is reduced to such a low level that it becomes difficult to accomplish the desulfurization process without operating the engine in the reduced-cylinder operating mode. The first threshold value is determined in advance through experiments, or the like. When the load of the engine 1 is reduced to the first threshold value or lower, burning of the fuel is stopped in half of the eight cylinders 4, namely, No. 1, No. 7, No. 4 and No. 6 cylinders or No. 2, No. 3, No. 5 and No. 8 cylinders, so that the fuel is burned at intervals of 180° crank angle. [0039] Since the exhaust temperature can be increased through the control as described above, the temperatures of the right-hand NOx catalyst 9 and left-hand NOx catalyst 10 are less likely to be reduced or prevented from being reduced even when the load of the engine 1 is reduced. Also, the amount of the fuel supplied to each of the cylinders in operation is increased so that the exhaust air/fuel ratio can be reduced. Accordingly, even in the case where the engine 1 shifts from a lowload condition to a high-load condition during the desulfurization process, the desulfurization process can be continued.

[0040] (2) If the engine 1 continues to be operated in the reduced- cylinder operating mode when the engine 1 no longer operates at the low load (i.e., the engine 1 comes out of the lowload condition) during the desulfurization process, the exhaust air/fuel ratio may become excessively rich, or the exhaust temperature may be excessively increased, or the torque of the engine may not be increased. To avoid these situations, the engine 1 is operated in an all-cylinder operating mode in which the fuel is burned in all of the cylinders 4. The engine 1 is switched from the reduce-cylinder operating mode to the all-cylinder operating mode when the load of the engine 1 becomes equal to or higher than a second threshold value that is higher than the above -indicated first threshold value.

[0041] Since the exhaust air/fuel ratio is reduced when the engine 1 is switched to the all-cylinder operating mode, a sufficiently high exhaust temperature required to accomplish the desulfurization may not be provided if the engine 1 is switched to the all-cylinder operation at, for example, the first threshold value. Therefore, the engine 1 is switched to the all-cylinder operating mode when the load reaches the second threshold value that is higher than the first threshold value. The second threshold value is set in advance, through experiments or the like, to a value (load) at which the desulfurization process can continue to be performed upon switching from the reduce-cylinder operating mode to the all-cylinder operating mode.

[0042] (3) In the case where there arises a need to perform a desulfurization process when the engine 1 runs at a low load, the engine 1 is operated in the reduced-cylinder operating mode. Here, the need to perform the desulfurization process arises when, for example, the amount of a sulfur component stored in the right-hand NOx catalyst 9 or left-hand NOx catalyst 10 exceeds a specified amount. The amount of the sulfur component stored in the right-hand NOx catalyst 9 or left-hand NOx catalyst 10 may be calculated based on the amount of fuel consumption, the output signal of a NOx sensor, the running distance of the vehicle, and so forth.

[0043] In this case, too, burning of the fuel is stopped in half of the cylinders 4, and combustion is caused to take place at intervals of 180° crank angle. In this manner, the exhaust temperature can be increased, and therefore, the temperatures of the right-hand NOx catalyst 9 and left-hand NOx catalyst 10 can be increased. Also, the amount of the fuel supplied to each of the cylinders in operation is increased, so that the air/fuel ratio of the exhaust gas can be reduced. Thus, even when the engine 1 runs at a low load, the desulfurization process can be carried out.

[0044] (4) When the desulfurization process is completed while the engine 1 is running at the low load, it is no longer necessary to continue operating the engine 1 in the reduced-cylinder operating mode. In this case, therefore, the engine 1 is immediately switched to the all-cylinder operating mode.

[0045] Next, the control flow of the desulfurization process according to the present embodiment will be explained. FIG. 3 is a flowchart illustrating a control routine of the desulfurization process according to the present embodiment. The routine of FIG. 3 is repeatedly executed at certain time intervals. [0046] In step SlOl, it is determined whether a desulfurization process for regenerating the right-hand NOx catalyst 9 or left-hand NOx catalyst 10 from sulfur poisoning needs to be performed. This determination is made by determining whether the amount of the sulfur component stored in the right-hand NOx catalyst 9 or left-hand NOx catalyst 10 exceeds the specified amount, as described above.

[0047] If an affirmation decision (YES) is made in step SlOl, the control proceeds to step S102 in which the desulfurization process is performed. If a negative decision (NO) is made in step SlOl, on the other hand, there exists no need to perform desulfurization, and therefore, the routine of FIG. 3 is once finished.

[0048] In step S102, it is determined whether the load of the engine 1 is equal to or lower than the above-mentioned first threshold value. Namely, it is determined in this step whether the engine 1 needs to be operated in the reduced-cylinder operating mode so as to perform the desulfurization process.

[0049] If an affirmative decision (YES) is made in step S102, the control proceeds to step S 103 to operate the engine 1 in the reduced-cylinder operating mode. If a negative decision (NO) is made in step S102, on the other hand, the control proceeds to step S 107 to operate the engine 1 in the all-cylinder operating mode.

[0050] In step S103, the desulfurization process is performed while the engine 1 is operated in the reduced-cylinder operating mode. At the same time, the exhaust values 15 of the cylinders 4 in which the fuel is not burned are kept in the closed positions. In the present embodiment, the ECU 13 that performs the desulfurization process while operating the engine 1 in the reduced-cylinder operating mode corresponds to the desulrurizing device of the present invention.

[0051] In step S104, it is determined whether the load of the engine 1 is equal to or higher than the above-mentioned second threshold value. Namely, while the desulfurization process is performed with the engine 1 operating in the reduced-cylinder operating mode, it is determined whether the load of the engine 1 has been increased to such a level that eliminates the need to operate the engine 1 in the reduced -cylinder operating mode.

[0052] If an affirmative decision (YES) is made in step S104, the control proceeds to step S107 to switch the engine 1 to the all-cylinder operating mode. If a negative decision (NO) is made in step S104, on the other hand, the control proceeds to step S 105 to perform the desulfurization process while continuing the operation of the engine 1 in the reduced-cylinder operating mode.

[0053] In step S105, it is determined whether the desulfurization process is completed, namely, whether the sulfur component has been released from the

NOx catalyst 9, 10. For example, it is determined that the desulfurization process is completed when the desulfurization process has been performed over a certain period of time.

[0054] If an affirmative decision (YES) is made in step S105, the control proceeds to step S106. If a negative decision (NO) is made in step S105, on the other hand, the control returns to step S104 to perform the desulfurization process while continuing the operation of the engine 1 in the reduced-cylinder operating mode.

[0055] In step S106, the desulfurization process is finished, and the engine 1 is operated in the all-cylinder operating mode. If the engine 1 has been operated in the re duced-cy Under operating mode, opening and closing of the exhaust valves 15 of the cylinders 4 in which combustion has been stopped are re-started.

[0056] In step S107, the desulfurization process is performed while the engine 1 is operated in the all'cylinder operating mode. If the engine 1 is switched from the reduce-cylinder operating mode to the all-cylinder operating mode, opening and closing of the exhaust valves 15 of the cylinders 4 in which combustion has been stopped are re -started.

[0057] In step S108, it is determined whether the load of the engine 1 is equal to or lower than the first threshold value. In this step, substantially the same operation as that of step S102 is performed.

[0058] If an affirmative decision (YES) is made in step S108, the control proceeds to step S103 to perform the sulfurization process while operating the engine 1 in the reduced-cylinder operating mode. If a negative decision (NO) is made in step S108, the control proceeds to step S109 to perform the desulfurization process while continuing the operation of the engine 1 in the all-cylinder operating mode.

[0059] In step S 109, it is determined whether the desulfurization process is completed. In this step, substantially the same operation as that of step S105 is performed.

[0060] If an affirmative decision (YES) is made in step S 109, the control proceeds to step S106 to finish the desulfurization process. If a negative decision

(NO) is made in step S109, on the other hand, the control returns to step S108 to perform the desulfurization process while continuing the operation of the engine 1 in the all-cylinder operating mode.

[0061] As explained above, according to the present embodiment, when the desulfurization process needs to be performed while the engine 1 is running at a low load, the engine 1 is operated in the reduced-cylinder operating mode so as to reduce the exhaust air/fuel ratio and increase the exhaust temperature, to thereby permit the desulfurization process to be performed. Thus, the desulfurization process can be performed over an expanded operating region of the engine, under various running conditions. Also, the amount of the fuel consumed in the desulfurization process can be reduced, resulting in improved fuel efficiency.

[0062] In the V-type eight-cylinder engine in which combustion successively takes place at least once at intervals of 90° crank angle, combustion is caused to take place at intervals of 180° crank angle when the engine is operated in the reduced-cylinder operating mode, and therefore, the engine can be operated with high stability. It is, however, to be understood that the reduced-cylinder engine operation is not necessarily performed in the V-type eight-cylinder engine, but may be performed in other types of internal combustion engines having other arrangements of cylinders so as to reduce the exhaust air/fuel ratio and increase the exhaust temperature. The engine having another arrangement of cylinders can also be operated with stability if the fuel is burned at equal intervals.

[0063] When the engine 1 is operated in the reduced-cylinder operating mode, the exhaust valves 15 of the deactivated cylinders 4 in which no combustion takes place is burned are kept closed by the variable valve actuating mechanism 11. This arrangement will avoid an otherwise possible situation where the exhaust air/fuel ratio is increased and/or the exhaust temperature is reduced due to air discharged from the deactivated cylinders 4 through the exhaust valves 15.

Claims

CLAIMS:

1. An exhaust emission control system of an internal combustion engine having a plurality of cylinders and capable of operating in a reduced-cylinder operating mode in which a fuel is burned in a reduced number of the cylinders, comprising: a storage-reduction type NOx catalyst operable to adsorb and store NOx and reduces the stored NOx in the presence of a reducing agent; and a desulfurizing device that operates the engine in the reduced-cylinder operating mode when a desiilfurization process for regenerating the storage -reduction type NOx catalyst from sulfur poisoning is performed.

2. The exhaust emission control system of the internal combustion engine according to claim 1, wherein burning of the fuel is stopped in half of the cylinders when the engine is operated in the reduced-cylinder operating mode.

3. The exhaust emission control system of the internal combustion engine according to claim 2, wherein: the internal combustion engine is a V-type eight- cylinder engine in which the fuel is burned at least once at intervals of 90° crank angle; and the desulfurizing device stops burning of the fuel in a first half of the cylinders while permitting the burning in a second half of the cylinders at intervals of 180° crank angle when performing the desulfurization process.

4. The exhaust emission control system of the internal combustion engine according to any one of claims 1 through 3, further comprising a variable valve actuating mechanism that keeps exhaust valves of the cylinders in which burning of the fuel is stopped in closed positions when the desulfurizing device operates the engine in the reduced-cylinder operating mode.

5. The exhaust emission control system of the internal combustion engine according to any one of claims 1 through 4, wherein the desulfurizing device operates the engine in the reduced-cylinder operating mode when a load of the engine becomes equal to or lower than a first threshold value.

6. The exhaust emission control system of the internal combustion engine according to claim 5, wherein the desulfurizing device stops operating the engine in the reduced-cylinder operating mode when the load of the engine becomes equal to or higher than a second threshold value that is higher than the first threshold value during operation of the engine in the reduced-cylinder operating mode.